Acid assisted cold welding and intermetallic formation

ABSTRACT

A metallic composite solid, containing alloys and/or intermetallics, is formed by compacting at moderate pressure a mixture of powder particles, foils or sheets at a temperature close to room temperature, well below the melting temperature of the constituent components and without the addition of low melting metals such as mercury, indium or gallium acting as a sintering agent. This low temperature consolidation of the powder mixture is enhanced by having the surface oxide of the powder particles removed, prior to consolidation, and/or by coating the particles with an oxide-replacing metal such as silver or gold. The coating process may be replacement reactions, autocatalytic reduction or electrolytic reduction. The composite formation is assisted by the addition of a liquid acid such as fluoroboric acid, sulfuric acid, fluoric acid, adipic acid, ascorbic acid, or nitric acid. A preferred embodiment of the process for metal solid composite formation is a process for forming dental restorative materials at ambient temperatures and under pressure exerted by manual dental instrumentation.

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/133,316, filed Oct. 8, 1993 abandoned, which is in turn acontinuation-in-part of U.S. patent application Ser. No. 07/802,420,filed Dec. 4, 1991, which issued on Jun. 7, 1994 as U.S. Pat. No.5,318,746.

FIELD OF INVENTION

The invention relates to a process of consolidating metallic andintermetallic composite materials and a process of forming bulkintermetallics at ambient temperatures. In preferred embodiments, theinvention relates to processes for forming the metallic compositematerials as in situ dental restorations, high temperature materials,copper-tungsten and similar materials for thermal management, aluminidessuch as nickel aluminides, nickel titanium alloys for shape memoryeffects applications and titanium nickel tin alloys.

BACKGROUND

Powder metallurgy is a processing technique whereby very small diameterpowder particles are compressed into parts or shapes by a number ofmethods that include vacuum hot pressing, hot isostatic pressing (HIP),sinter hipping, hot forging, etc. These processes require the sequentialor simultaneous application of high temperature and pressure. Typically,the temperatures used in powder metallurgy are at an appreciablefraction of the melting point (T_(m)) of the compressed elements oralloys, usually above 0.8 T_(m). The pressures applied are often near orbeyond the yield point of the metals involved. In the case of metalfoils or sheets, consolidation is often done by hot roll bonding. One ofthe reasons for these severe conditions results from the need to breakup the naturally occurring oxide on the surface of the material, therebyenabling the surfaces of the powder particles (foils or sheets) to weldtogether at a sufficient number of contact points so as to provideadequate adhesion between the individual particles, sheets or foils.

Powder metallurgy is useful as an alternative to comelting appropriateamounts of metal constituent components in forming intermetalliccompounds. Intermetallic compounds have a great potential for a varietyof applications as a result of their specific properties such ashardness, high elastic moduli and oxidation resistance. On the otherhand, the inherent brittleness of intermetallic compounds severelycurtails their use in conventional thermomechanical processingoperations to form net shapes. As an alternative, powder technology isoften used for processing intermetallic compounds. The startingmaterials for this approach are pre-alloyed compounds that have beencomminuted by various methods into powder particles. The limitation ofthis approach is that it relies on compaction of powders which areinherently brittle and do not deform with ease. Compound formation,however, may also take place by solid state interdiffusion of mostlyductile constituent elements which can be compacted with relative ease.Mixtures of elemental metal powders, maintained in close mutual contactfor a sufficient length of time at the appropriate temperature,interdiffuse and form intermetallic compounds. In some situations,intermetallic compound formation is required, but exposure to elevatedtemperature has to be limited or avoided. An example of such a situationmay be the requirement for compound formation (for protheses or asdental restorative material) in a human body environment.

Intermetallic compound formation by interdiffusion of the constituentelements or extremely finely divided multi-phase solid formation bynon-compound forming and non-interdiffusing elements is favored when thestarting materials are in the form of a very small size particle powder.Such powders possess a large specific surface area, and hence, whenmixed, form a relatively large interface area between the differentconstituents. The generation of an interface area between the differentconstituents depends on the efficiency of the mixing technique and alsoon the nature and properties of the mixed powder particles.

Several mixing techniques are commonly used in order to maximize thecontact points (interface area) between particles of different kinds. Ifthe effect of particle properties on the outcome of the mixing processis neglected, prolonged mixing will tend to maximize the number ofcontact points between different particles by striving towards a randomdistribution of the particles of different kind. Many particleproperties such as particle size and shape, surface roughness, inaddition to electrostatic phenomena, promote segregation effects andthus reduce and curtail the homogeneous mixing of different powders.Thus, multimodal particle size distribution favors space filling andincreased density but also favors particle segregation. The mostcommonly used mixing technique relies on the tumbling-type blending ofthe powders. Ball milling is another technique that is used for mixingand also for reducing particle size. An extension of ball-milling is themechanical alloying technique that yields alloyed powder products fromelemental powder mixtures. Alloy or compound formation by ball millingis dependent on the kinetic energy input due to the rapidly rotatinghard balls impinging on the powder particles. Thus ball-milling leads tohigh local temperature increases.

Intermetallic compound formation at the interface of two metals inintimate contact is a documented phenomenon. In some instances, theformation of intermetallic compounds is beneficial, in others, itseffects may be detrimental. The formation of a new compound attemperatures below the melting point of the metals in contact relies oninterdiffusion effects in the solid state. In most binary combinations,ambient temperature is well below the melting temperature of theconstituent metals and, consequently, little or no compound formationtakes place at the interface. Notable exceptions to this are diffusioncouples in which one of the constituent metals, e.g. mercury or gallium,has a low melting point, below or close to room temperature. Anotherimportant group of binary combinations which shows room temperaturecompound formation, consists of a group I-B of the periodic table (Cu,Ag or Au) metal juxtaposed to a group III-A or IV-A (In, Sn or Pb)element. K. N. Tu et al., Jap. J. Appl. Phys. Suppl., Pt.1, 633 (1974).It is believed that room temperature compound formation in these systemsis related to fast diffusion behavior of the noble or near noblecomponent (the I-B elements) in the matrices of the group III-A or IV-Ametals. A. D. LeClaire, J. Nucl. Mat. 69 & 70, 70 (1978). Fast diffusionoccurs by virtue of the interstitial or partly interstitial diffusivityof the fast diffusing components, W. K. Warburton et al., "Diffusion inSolids, Recent Developments", Nowick and Burton (eds.), Academic Press,New York, 1975, p.172. It is noteworthy that interfaces between twocomponents, each of which respectively belongs to one ot the two classespreviously defined, are of common occurrence in electronic devices andit is not surprising, therefore, that such systems have been subject torelatively close scrutiny over the past years K. N. Tu, Ann. Rev. Mater.Sci., 15, 147 (1985). The quasi-totality of the room temperatureintermetallic compound formation studies in these systems has made useof the thin film configuration. This configuration yields samples with ahigh interface to total volume ratio permitting effective study ofcompound formation at the interface. The phase diagrams in most binarycombinations of this kind show the presence at room temperature ofseveral equilibrium intermetallic compounds, (Table I).

                  TABLE I                                                         ______________________________________                                        Number of intermetallic compounds that are present in binary                    systems containing noble metals in which room temperature compound           formation takes place.                                                                In             Sn    Pb                                              ______________________________________                                        Cu       3              2     0                                                 Ag               3            2             0                                 Au               4      4       2                                           ______________________________________                                    

                  TABLE II                                                        ______________________________________                                         Numberof intermetallic compounds that are present in binary                    systems other than those containing noble metals in which fast                 diffusion effects take place.                                                        Ti        Zr    Gd.sup.1   U.sup.2                                  ______________________________________                                        Fe        2         4     4          2                                          Co          5         5          7          6                                 Ni          3         8          7          6                                 Pd          8(10)     4          6          5(7)                              Pt          4(6)      3          8          4                               ______________________________________                                         .sup.1 A Gd matrix is taken as a prototype for lanthanide elements, as on     of the two components of a binary combination.                                .sup. 2 Uranium matrix is taken as a prototype for other actinide             elements, as one of the two components of a binary combination.               .sup.3 In parenthesis, the number of intermetallic compounds including        those stable at elevated temperatures or not yet well established.       

Fast diffusion effects are not restricted to the above-mentionedsystems. Other notable and relevant systems are combinations of an earlytransition metal element from the Group III-B (Sc, Y or a lanthanideelement, Th and U) or from Group IV-B, (Ti, Zr or Hf) with a latetransition metal from Group VIII (Fe, Co, Ni, Pd or Pt). Fast diffusionof the small size late transition metal elements in the matrix of theearly transition metal elements has been reported in the literature. Inthese latter systems, however, none of the constituent elements has amelting point even relatively close to room temperature. Thus, in spiteof fast interdiffusion, some exposure to intermediate temperatures isnecessary in order to achieve any significant intermetallic compoundformation within a reasonable time frame.

The formation of intermetallic compounds in even a relatively simplesystem such as two juxtaposed thin films, is a complex process. Itdepends on a number of variables such as the relative thickness of theindividual initial layers, the diffusion mechanisms and thediffusivities of the atomic species in the different layers beingformed, the nucleation characteristics of the various compounds, tomention just a few of the relevant parameters. It is not surprising,therefore, that in spite of the relatively large number of completedstudies, no clear picture emerges regarding the outcome of theinterdiffusion process in a thin film couple.

The thin film configuration, even though allowing an increase in therelative amount of compound to be formed at the interface, does not lenditself to the formation of bulk intermetallic compounds. Bulk formationof intermetallic compounds may be of both theoretical as well aspractical interest. Bulk formation at room temperature and ambientpressure is of interest if extraneous constraints preclude the use ofconventional processing and production methods, i.e. casting from themelt or diffusion assisted formation at elevated temperatures.

Another important use for powder metallurgy is its use in amalgams andrelated alloys. Metallic dental restorative materials used in dentalfillings, placed directly in tooth cavity preparations, can beclassified broadly into two classes, direct gold fillings and dentalamalgams (O'Brien, 1989; Phillips, 1991). Dental amalgams are metalliccomposites resulting from a reaction between mercury and variouspre-alloyed silver-tin-copper alloys. The mixing of mercury, which isliquid at ambient temperature, with the alloy in powder form takes placeimmediately prior to insertion in the dental cavity. The mixture,compacted into the cavity with dental instruments, consolidates into acohesive solid and hardens over a length of time. Dental amalgams aremuch harder than pure gold fillings, they display relatively highcompressive strength but are brittle and possess low transverse-rupturestrength.

Amalgams and related alloys have been incorporated into a variety ofcommercial applications and thus a number of processes for producingsuch amalgams are known. For example, U.S. Pat. No. 4,664,855 disclosesa universally employed process that triturates elemental metals orintermetallic alloys, in the form of comminuted filings or atomizedspherical powders, with the sintering agent mercury and compacts theresulting amalgam into a uniform, consolidated metallic composite. Theprocess may be considered a combination of liquid phase and reactivemetal sintering. The finely comminuted metallic or intermetallic powdersreact with the Hg and when pressure is applied to the reaction product,form a compact, high density mass. U.S. Pat. No. 3,933,961 discloses aprocess for preparing a pre-weighed alloy tablet of uniform weight thatis then triturated with a weighed quantity of Hg to form a traditionalamalgam alloy.

The mercury content of dental amalgams has been a recurring source ofconcern because of the health and environmental hazards associated withits presence. Many aspects involved in the use of dental amalgams suchas the various hazards, the possible substitute materials, theiradvantages and drawbacks, the economical considerations that areinvolved have been reviewed and discussed extensively in variouspublications as for example: Effects and Side-effects of DentalRestorative Materials", Adv. in Dental Res. 6:, September 1992; JADAVol. 122, August 1991, papers p.54-61, p.63-65, p.67-71, p.73-77; JADAVol. 125. April 1994, papers p. 381-387, p. 392-399).

Gold, either in the form of foils, powder or pellets can be used insteadof mercury containing dental amalgams in direct filling. Prior to itscondensation, pure gold, in all its forms, has to undergo a degassingprocedure to desorb any adsorbed layers that might impede or preventconsolidation into a cohesive solid. Degassing is achieved by exposingthe filling material to elevated temperature just before its insertionin the dental cavity. Clean gold surfaces and other noble metalsurfaces, devoid of adsorbed layers, cold-weld under moderate pressureto form cohesive solids. Pure gold fillings are malleable and ductileand display high values of transverse rupture strength but low values ofhardness and compressive strength.

SUMMARY OF THE INVENTION

One of the objectives of the present disclosure is to present a novelapproach to low temperature compound formation in large quantities,using a method which takes advantage of the interdiffusion processesoccurring at relatively low temperatures and ambient pressure.

In one aspect, this invention pertains to a metallic compositerestorative material formed from a mixture of elemental metals, alloysand/or intermetallic compounds that have been given an appropriatesurface treatment. The invention further pertains to a process forpreparing the metallic composite by compacting the surface treatedmixture of elemental metal powders, without adding a liquid metallicagent, such as mercury, to form a solid, cohesive metallic compositebody, in situ. Compaction can be performed at ambient temperature, belowthe melting points of the surface treated powders present in themixture, under pressure sufficient to form a uniform metallic composite.

Although not wishing to be bound by any one theory, the presentinventors believe that the invention rests on several physicalprinciples and specific findings that can be briefly described asfollows:

i) Cold-welding takes place across appropriately treated metal surfaces,from which oxide or adsorbed gas layers have been removed. The presentinvention comprises treating the metal surfaces by immersion in reducingagent such as a mild or dilute acid to efficiently surface clean a noblemetal (e.g. silver). Acid assisted consolidation of silver particlestakes place at room temperature under moderate pressure to yieldcohesive solids. As an extension of this finding, the present inventorshave found that intermetallic compound particles or metal particlesother than a noble metal, when coated with an external noble metal, ormore noble metal layer readily undergo acid-assisted consolidation.

Noble metals are metals, as for example, silver, gold, platinum andpalladium, which do not readily oxidize in air. Thus, more noble metalsare those metals which have a more positive Standard Reduction Potential(SRP) in the electrochemical series (Handbook of Chemistry and Physics,page D-155, 61st Ed., 1980-81).

(ii) Metallic composite materials can be prepared from powder mixturesthat include a soft ductile component, preferably a noble metal, e.g.silver, and hard intermetallic compound components that have been coatedwith a noble metal such as for example silver. Acid-assistedconsolidation of such mixtures yields composites whose mechanicalproperties depend on the ratio of the soft to hard component.

(iii) Bulk quantities of intermetallic compounds can be prepared atambient temperature if the interface area between the two metalcomponents that interdiffuse is sufficiently large. By taking advantageof a coating process, as for example, but not limited to, electrolyticdeposition from a fluidized bed, physical deposition processes and animmersion deposition process whereby a more noble metal, e.g. silver,deposits from a solution in which a less noble metal, e.g. tin, has beenimmersed in the form of solid powder particles. A huge interface areabetween the more noble and less noble component equal to the surfacearea of the less noble (e.g. tin) particles is thereby produced. In sucha system, bulk quantities or even complete transformation intointermetallic compound can be obtained. Such a material, when mixed withor containing a sufficient amount of a noble metal, will readily undergoacid assisted consolidation and yield a cohesive solid whose mechanicalproperties such as transverse rupture strength, compressive strength andhardness can be adjusted by varying the relative amount of the variouscomponents.

The metallic composites prepared by making use of the above are usefulas dental restorative materials, specifically as mercury-freealternatives to dental amalgams.

In summary, this invention pertains to 1) the synthesis of bulkquantities of intermetallic compounds (A_(m) B_(n)), 2) the synthesis offinely dispersed two-phase alloys and 3) synthesis of metallic matrixcomposites (MMC) at temperatures significantly lower than the meltingtemperature of the constituent elements, 4) application of immersioncoated metallic powders to synthesize composites, 5) application of fastinterdiffusion couples to promote compound formation making use of acidassisted consolidation techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows values of transverse rupture strength of consolidatedsilver powders. The figure shows the effect of the compression pressureand the significant increase in rupture strength achieved byacid-assisted consolidation (AAC).

FIG. 2 shows the effect of the acid concentration on the density andcompressive strength of silver powders that have undergone acid-assistedconsolidation by impact.

FIG. 3 shows two superimposed x-ray diffraction spectra of a sampleprepared according to the procedure described in Example 2. Thesespectra illustrate the compound formation at ambient temperature.

FIG. 4 are x-ray diffraction spectra of a consolidated sample ofAg-coated Sn powder (Example 3) taken at different times afterconsolidation. Spectrum 4a was started 30 min after consolidation,spectrum 4b was taken 8 days later.

FIG. 5 is a micrograph of a hand consolidated sample described inExample 3, the white areas represent the residual Sn, the gray areas theintermetallic compound formed at the Sn--Ag interface.

FIG. 6 are x-ray diffraction patterns of a Ag-coated nearlystoichiometric Ag₃ Sn powder (Example 4) taken at different times afterconsolidation. Spectrum 6a was taken after consolidation, spectrum 6b,64 h later. The sample was kept at 37° C.

FIG. 7 is a micrograph of the sample described in Example 4, the whiteareas represent excess Ag, the gray areas represent mainly initialspherical intermetallic compound particles and also the additionalcompound formed at the Ag--Ag₃ Sn interface.

FIG. 8 are x-ray diffraction patterns of a consolidated Ag-coatedatomized Ag--Sn alloy powder sample (Example 5). The initial compositionof the alloy was 60 wt. % Ag and 40 wt. % Sn, namely a two phase, Ag₃ Snand Sn alloy. Spectrum 8a was taken 19 h after consolidation andspectrum 8b, 119 h later.

FIG. 9 is a micrograph of the sample described in Example 5, the whiteareas represent excess Ag, the gray areas represent mainly initialspherical intermetallic compound particles and also additional compoundformed at the Ag--Ag₃ Sn interface.

FIG. 10 are diffraction patterns of a consolidated Ag-coated atomizedAg--Sn alloy powder sample, similar to that described in Example 6, butcoated by a different chemistry. FIG. 10a shows the diffraction patternof the initial Ag--Sn alloy, prior to coating. FIG. 10b shows an x-raydiffraction pattern of the sample having undergone a procedure accordingto the present invention.

FIG. 11 is a micrograph of the sample prepared according to he methoddescribed in Example 6 that yields silver-coated Ag₃ Sn particles. Thereis continuous bright Ag coating surrounding each, (mostly spherical),well defined Ag--Sn alloy particles. The black dots are etch pitslocated at the excess free Sn sites. Some pores are also visible at thejunction points of several particles.

FIG. 12 is a micrograph of a consolidated, atomized composite dentalalloy, consisting of Ag-coated, atomized Ag₃ Sn particles, atomizedCu--Ag alloy particles embedded in a matrix consisting of free Agparticles (Example 7).

FIG. 13 shows the diffraction patterns of a consolidated mixture ofindium particles immersion coated with silver (Example 8). (a) 1 h afterconsolidation wherein the sample consists of a multi-mixture of phases,AgIn₂, Φ-phase, ζ-phase, stable under equilibrium conditions mostly atelevated temperature, excess In and the solid solution of In in Ag. (b)after a lengthy (142 d) stay at room temperature wherein the sampleconsists of approximately 90% Φ-phase and 10% Ag₉ In₄, γ-phase.

FIG. 14 shows diffraction patterns of a consolidated mixture of tinparticles immersion coated with gold (Example 9). The sample consistsessentially of the δ-AuSn compound, a small fraction of AuSn₂, theε-phase, and some excess Au.

FIG. 15 is an optical micrograph of a Cu--W bi-phase composite,consolidated under dilute fluoroboric acid as described in Example 10.

FIG. 16 is a low magnification scanning electron micrograph of a crosssection of a tooth containing a filling consisting of a mixture ofelectrolytically silver-coated Ag₄ Sn that had been mixed with elementalsilver and elemental tin particles and immersion coated with silver.This mixture was hand-consolidated with conventional dental instruments.

FIG. 17 is an optical micrograph of the dentin-filling interface of thefilling shown in FIG. 16.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In one aspect the present invention pertains to a process for formingmetallic restorative metallic composites or alloys which circumvents theneed for a metallic liquid sintering agent such as mercury, indium, orgallium or other low melting-point metallic alloys and relies on analternative to the so-called amalgamation process or to the gold fillingprocess.

This process relies on a combination of liquid acid assistedconsolidation and fast diffusion effects that take place in some metalsand their alloys.

Liquid acid assisted consolidation (AAC) enables cold-welding of loosepowder particles, foils or sheets under moderate pressure at ambienttemperature into a cohesive solid. The present inventors have found thatsuch cold-welding, that takes place across particle surfaces that are incontact, can be made possible by appropriately surface treating theparticles as an alternative to traditional high temperature exposure,commonly used to cold-weld gold surfaces. Specifically, in accordancewith the present invention, noble metals (or more noble metals), e.g.,silver, gold, palladium and copper, cold-weld after having undergonesuitable surface treatment by immersing the metal provided in particles,foils or sheets in a reducing media based on water, alcohol or oil thathas the property of removing surface oxides. Preferred reducing mediainclude mild or dilute acids, in order to remove the surface oxidelayers. Suitable acids for use in the surface treatment according tothis invention include, but are not limited to organic acids such as forexample acetic acid or inorganic acids, as for example, fluoroboricacid, sulfuric acid, hydrofluoric acid, hydrochloric acid, sodiumasorbate, potassium asorbate, citric acid, adipic acid, ascorbic acid,sulfamic acid with or without ammonium bifluoride and nitric acid.Suitable concentration for the acid for use in the invention are fromabout 1% to about 30%. The nature and concentration of the reducingmedia depends on the nature of the application. For example, in dentalapplications, fluoroboric acid is preferred in concentrations rangingfrom about 2 to about 10% by volume, with about 2.5% by volume beingmost preferred.

The surface treatment, essentially an electrochemical treatment, whichcan be carried out at about ambient temperature, comprises thoroughcleaning of particulate, metal or pre-alloyed intermetallic compound,and removal of the surface oxide layers by immersion in the acidsolution.

The present invention should not be construed to be limited to reducingthe surface oxides solely by reducing media. Oxides on particles canalso be, for example, electrolytically reduced even in oxidizingsolutions by applying a negative potential. In particular, solutionscontaining sulfate can be used to reduce the oxide on iron by applying anegative potential even though the oxide will reform once the potential(voltage) is removed.

The present invention further pertains to a process based on theaforementioned embodiment wherein non-noble metals, as for example, butnot limited to, Ga, In, Ir, La, Re, Rh, Ru, Sn, Ti, Y, Zn, Nb, Mo, Ta,Sc, Hf, Th, Ce, Pr, Nd, Sm, Gd, TB, Gy, Ho, Er, Tm, Yb or Lu; orintermetallic compound particles are cold-welded by providing them withan external oxide-replacing metal coating of a more noble metal, as forexample such as Ag, Au, Pd, Fe, Ni, Cu, Co or Pt. The process comprisesimmersing the non-noble metals or intermetallic compound particles in asolution containing an electrolyte. The electrolyte is for example, butnot limited to, at least one sulfamate, iodide, cyanate, nitrate,pyrophosphate, fluoroborate or sulfide salt of the oxide-replacing (morenoble) metal. An electrolytic or coating process is conducted orreplacement reactions are allowed to form the coating and subsequentlyseparating by filtration, centrifuge, evaporation or other suitablemeans, the coated powder from the electrolyte solution. The coatedpowder is then cleaned, rinsed and immersed in a liquid acid solutionand consolidated into a cohesive solid. The particles are coated inorder to ensure that each particle has an external surface from whichany oxide layer can be removed efficiently so that it readily undergoescold-welding under moderate pressure. Contrary to known "cold-welding"of gold fillings discussed in the background above, no exposure to hightemperature is required prior to consolidation of the coated particles.

Other methods for removing oxides from the metal surfaces and preventingfurther oxide formation, other than the electrochemical method describedabove, may also be used. For example, gas plasmas with inert andreducing atmospheres, such as the forming gas (5% hydrogen and 95%nitrogen), also remove surface oxides from metals. Similarly, vapordeposition, sputtering or mechanical plating will coat the powderparticles with protective layers of Ag, Au or a related alloy which hasnon-tenacious oxides.

As a direct filling alloy for dental applications the powders, foils andsheets may be subjected to a cathodic and/or anodic treatment which isfollowed by an acid treatment. The thus treated wet powders, foils andsheets are consolidated into a net shape. The cathodic and/or anodictreatment may be in combination with a rinsing step or may be eliminatedin certain circumstances. Depending on the particular elemental,metallic, intermetallic or alloy used, there may be an optimalconcentration of the acid. Preliminary results are shown in FIG. 2,wherein the density and the strength are measured in compression as afunction of the acid concentration.

The liquid present between the suitably coated powder particles seepsout from between the particles (foils or sheets) during the acidassisted consolidation. The liquid provides a very important secondarybenefit in that very small powder particles are constrained under thesurface of the liquid so that they can be handled more safely. Ininstances where the particles are used as in situ dental restorations,the patient will not inhale them, Another benefit of the liquid islubrication of the compressed particles.

The "non-amalgamation" process described herein relies on fast-diffusioneffects that take place in some metallic systems. Fast-diffusion asdiscussed above, is attributed to the ability of some diffusing speciesto penetrate interstitially into a host matrix. Diffusion takes placevia an interstitial mechanism instead of the usual vacancy mechanismcharacteristic of diffusion in most metal systems. Interstitialdiffusion is usually much faster than diffusion governed by themetal-vacancy mechanism. The fast-diffusing species are small-size,low-valence constituents (e.g., Ag) which diffuse rapidly within thehost matrix of a large-size, usually high valence component (e.g. Sn).Similar behavior has been observed for mono-valent metals (e.g., Cu, Agand Au) and some transition metals (e.g., Fe, Co, Ni and Pd) asdiffusing species in Group IV-B metals (Sn, Pb), Group III-B metals (In)and some early transition metals (e.g., Y, La, Ti and Zr) as metalmatrices.

Fast diffusion effects are relevant to the "non-amalgamation" insofarthat they promote intermetallic compound formation at ambienttemperature. When the two components of such a fast-diffusing system areput in physical contact, interdiffusion occurs. For example, theintermetallic compounds Ag₃ Sn and Ag₄ Sn may be prepared byinterdiffusion of silver and tin. Silver will diffuse rapidly into tinand also tin into silver (via grain-boundaries), until the respectivesolubility levels, (of silver in tin and tin in silver) are reached. Atthe solubility limit, the intermetallic compounds Ag₃ Sn and Ag₄ Snform. The amount of intermetallic compound that forms at the interfaceof mixed powders depends on the contact area between the tin and silverparticles. The amount of intermetallic compound formed near thatinterface at about body temperature, preferably about 37° C., islimited. Typically, the amount of intermetallic formed does not exceedthe layer width of the interface region of about 0.5 to 1 μm.

AS mentioned above, mixing elemental silver and tin powders will lead tothe formation of a certain amount of Ag--Sn intermetallic compounds. Inorder to increase the volume fraction of the Ag₃ Sn and Ag₄ Sn compoundin the final product, comminuted amounts of Ag₄ Sn and/or Ag₃ Sn may bepreferably added to an initial mixture of silver and possibly tin. Theresulting initial mixture preferably consists of a certain weightfraction of the intermetallic compound with elemental silver, and/orsilver based alloys. Elemental tin may also be added to the initialmixture. The relative fraction of the powdered, pre-alloyedintermetallic Ag₃ Sn and /or Ag₃ Sn in the mixture may preferably rangefrom 0 to 70 percent.

The formation at ambient temperature of intermetallic compounds bydirect interdiffusion of silver and tin is illustrated in Examples 2 to7. Fast interdiffusion and resulting intermetallic compound formationare not restricted, as mentioned, to the Ag--Sn system. Other possiblecombinations of fast interdiffusing metals will give rise to ambienttemperature compound formation, as shown in Examples 8 and 9.

The properties of the consolidated material are determined by theproperties of the starting materials incorporated into the mixture, bythe relative amount of each component, by the surface treatment appliedand by the details of the consolidation procedure that was used. Thus,increasing values of pressure applied for consolidation increase thedensity, compressive strength and rupture strength of the final product.The condensability, namely its propensity to transform from a loosepowder or slurry into a cohesive solid, and the transverse rupturestrength are increased by increasing the unalloyed metal, e.g. silver,content of the mixture. Increased levels of hardness and compressivestrength can be attained by incorporating into the mixture fractions ofpre-alloyed intermetallic compounds which, in general, are much harderthan the unalloyed metals. These compound particles can be spherical orlathe cut pre-alloyed silver-tin intermetallic compounds, for example,with various additions of other alloying elements. Pre-alloyedintermetallic compounds containing other components can also beconsidered. Moreover, non-metallic hard compounds such as oxide, carbideor nitride particles in the form of high-strength structural whisker,particulate or fiber additives can also be incorporated in the mixture.Such additives may include, but are not limited to, alumina powder,silicon carbide powder, graphite, diamond, sapphire, or the like. Otherwhisker, fiber or particle additives are within the scope of theinvention. In addition, or instead of the pre-alloyed intermetalliccompounds or other compounds, a hard intermetallic compound fraction canbe formed in-situ within the consolidated mixture. In-situ formationimplies compound formation of bulk quantities of intermetalliccompounds, as previously described, after the mixing of the powdercomponents has taken place.

Metal combinations which give rise to intermetallic compound formationinclude, but are not limited to, members of the group consisting of Au,Ag, Fe, Pt, Pd, Ni, Co and Cu as a first component in combination with amember selected from the group consisting of Ga, In, Ir, La, Re, Rh, Ru,Sn, Ti, Y, Zn, Nb, Mo, Ta, Sc, Hf, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er,Tm, Tb, Lu, U, Th and W. A preferred metal combination that gives riseto intermetallic compound formation contains a first elemental metal,Sn, and a second elemental metal, Ag. A preferred Ag:Sn ratio of the twometals is from 4:1 to about 3:2. More preferably, the Ag:Sn ratio isfrom 4:1 to 3:1.

In order to ensure adequate condensation of a powder mixture thatincludes pre-alloyed intermetallic compound or other hard particles intoa cohesive solid, it is preferred that these particles be provided withan external coating that is prone to cold-welding. Preferably thiscoating should be a more noble or noble metal, e.g., silver coating. Aspreviously mentioned silver surfaces readily undergo acid assistedconsolidation.

The external surface coating can be provided by any technique forsurface coating of powder particles. These include coatings from agaseous or from a liquid phase. Gaseous coatings include but are notlimited to fluidized bed, vacuum evaporation, sputtering, plasmaassisted and other techniques. Coatings from a liquid phase include butare not limited to electrolytic, as for example electrolytic coatingfrom a fluidized bed onto particle or fibers; immersion or substitutiondeposition. One example of a fluidized bed coating technique isdescribed in a copending application of some of the present inventorsentitled "Electrochemical Fluidized Bed Coating of Powders", herebyincorporated herein in its entirety by reference, filed concurentlyherewith and having Attorney Docket No. 53917.

For example, FIG. 12 represents a silver-coated Ag₃ Sn powder mixed withelemental Ag, Sn and atomized Cu--Ag alloy particles and thenconsolidated. The Ag₃ Sn particles have a thin Ag coating. Thesilver-coated Ag₃ Sn particles bond well to the free Ag particles whichmake up most of the Ag matrix. The atomized Cu--Ag particles provideincreased hardness to the material.

After the elemental metallic, alloy, intermetallic or other hardconstituent powder particles are coated with a more noble metal, as forexample, silver, gold, or copper, a consolidated composite solid bodysuch as a dental restoration may be formed, in situ (e.g., in a dentalcavity or mold for a commercial press), by compacting the wet surfacetreated mixture of the oxide-free and coated powders, without adding anymetallic liquid sintering agent. Examples of compacting include, but arenot limited to, die press and sinter, roll bonding, extrusion, "hipping"(hot isostatic pressing), "hot pressing or cold pressing", or compactingwith normal or modified dental instruments in situ in a patient's mouth.

Immersion or substitution deposition of silver or gold is an extremelyuseful method, in the context of the present invention, in order toprovide a very finely dispersed deposit of silver or gold. According tothis technique, a metallic powder, consisting either of elemental metal,alloy or intermetallic compound particles, is immersed in a aqueoussolution of a more noble metal. The less noble metal constituents of theimmersed solid powder undergo partial oxidative dissolution in thesolution, concurrently with the reduction and deposition of the morenoble metal from the solution. This method allows the creation of alarge interface area between the residual, partly dissolved solidparticles and the more noble, metallic deposit from the solution. Theadvantages of this method of deposition of the noble metals silver orgold are twofold. Firstly, a pure silver or gold deposit is provideduniformly through the mixture. Such a deposit will provide the requiredinterface material that readily cold-welds by acid assistedconsolidation. Secondly, if the initial solid particles, after theirpartial oxidative dissolution, still contain metallic elements which aredefined as appropriate host metal for fast diffusion, e.g., Sn, theconditions for the formation of intermetallic compounds by fastinterdiffusion are met. Moreover, this method provides a mixture ofpowders, i.e. the initial solid powder e.g., Sn-containing particlescoated with the fine Ag deposit, that displays a large Sn--Ag interfacearea. Maximum interface area will, of course, be achieved by having eachindividual tin (or tin-containing compound) particle coated by a silverlayer. At each tin-silver interface, interdiffusion takes place leadingto the formation of additional Ag₄ Sn compound until the compound layerthickness acts as a barrier to further interdiffusion. A high compoundcontent of the product, prepared according to this approach, is shown inExamples 3, 5, 6 and 7. In all these cases, the powder particles wereprovided with a silver environment by having the silver deposit from thesolution. In Example 3, the particles are pure Sn particles, in Examples5, 6 and 7 they are silver-tin compound particles with about 18 wt. %free tin content. The diffraction patterns taken after different timeintervals after the samples had been consolidated show an appreciableincrease of the compound Ag₄ Sn+Ag₃ Sn fraction in the final sample.Even in Example 4, in which a nominally stoichiometric compound wassilver coated, an increase of the compound content was observed afterthe sample had been kept at 37° C. for 64 h. In this case, in spite ofthe nominal stoichiometric composition, the free Sn (which originatedfrom the fast cooling of the atomized powder) reacted with silver andyielded some additional Ag₄ Sn+Ag₃ Sn compound fraction.

Compound formation by this method is not limited to powders in which alarge Ag--Sn interface is created. Example 8 illustrates compoundformation in the Ag--In system, Example 9 illustrates compound formationin Au--Sn systems, Example 12 illustrates compound formation in Cu--Snsystems, respectively.

An in situ dental restoration, as for example dental fillings, is apreferred application of the compacted alloy thus formed. In oneembodiment, commercially available amalgam powder packages (without theHg component) may be surface treated according to the process of theinvention, mixed with appropriate other powders and subsequentlycompacted to form a consolidated alloy. These commercially availablepowders may typically consist of combinations of intermetallics whichmay contain copper and/or zinc (these intermetallics typicallyapproximate the Ag₃ Sn compound). In another embodiment of theinvention, Ag and pre-alloyed intermetallic compounds at or close to theAg₃ Sn composition coated with silver or gold are compacted at bodytemperature. In still another embodiment of the invention, elemental tinpowder, silver-coated, is mixed with silver and/or silver-copper alloypowder particles and compacted at body temperature.

A preferred coated powder used in dental applications is Ag₃ Sn and/orAg₄ Sn and a preferred elemental powder is Ag, with a preferred overallAg:Sn atomic ratio ranging from 3:1 to about 8:1.

Pre-alloyed intermetallics, subsequently combined with elemental silverand/or tin particles, may consist of a compound having a weight ratio ofAg:Sn ranging from about 5:1 to about 3:1. In these compounds an excesstin may be locally present. This excess can be compensated for byincreasing a weight ratio of the elemental silver to intermetalliccompound.

The in situ formation of the alloy preferably occurs at a temperaturebelow the melting point of the coated powders and under an appliedpressure. Exemplary ranges of temperature and pressures under which thealloys may be formed include, but are not limited to, from 20° C. toabout 100° C., and from 20 MPa to about 400 MPa, respectively. Apreferred temperature for the consolidation of the composite for dentalrestorative purposes is about body temperature. A preferred pressure isabout 200 MPa or approximately the pressure exerted by ordinary dentaltools.

It is preferred to optimize the space filling ability of the powdermixture. It is also preferred to increase the contact area between thepowder particles. Intermetallic compounds of the invention may haveexemplary equiaxial particle sizes ranging from 0.5 μm to about 100 μm.Preferably, the particle sizes of the powders range from about 0.5 μm toabout 40 μm. Space-filling is improved by having multi-modal particlesize distributions. Preferably, the mixture should consist of particlesin the 30 to 40 μm size range admixed with particles in the 2 to 10 μmsize range.

In yet another embodiment of the invention, the process can be appliedto systems in which neither solid solutions nor intermetallic compoundsare formed. An alloy consisting of two such metals will display amixture of two phases, associated with the two components. Thehomogeneity of such a mixture will depend on the initial particle sizeand the conditions under which mixing was carried out. In many instancesthe properties of such alloys follow the rule of mixtures law and scalewith the respective concentration of the components. An immersioncoating process can be taken advantage of in order to ensure ahomogeneous distribution of the two components at a scale which isdetermined by the particle size of the component that is added in theform of powder to the solution. Thus, physical and electronic propertiescan be engineered as for example, thermal diffusivity, thermalconductivity, the coefficient of thermal expansion, magnetic propertiesand electronic properties can be custom-tailored to specific situationsby using the process according to the present invention. Engineeringproperties of coated particles by controlling the relative volumefractions of the particulate material and the coating material isdescribed in a copending patent application of one of the presentinventors (U.S. patent application Ser. No. 08/102,532, filed Aug. 4,1993, now U.S. Pat. Nos. 5,453,293, 5,601,924 and 5,614,320, issued Sep.26, 1995, Feb. 11, 1997 and Mar. 25, 1997, respectively which is acontinuation of Ser. No. 07/731,809 filed Jul. 17, 1991 ABN), entitled"Methods of Manufacturing Particles and Articles Having EngineeredProperties and Applying Coatings Having Engineered Properties toArticles", hereby incorporated herein in its entirety by reference.

The previously described immersion or substitution deposition of metalsis used to form the uniform and fine distribution of the two components.The electrolyte containing the more noble component has to be able todissolve the less noble component initially immersed as a solid and alsoto dissolve any protecting oxide layer present on its surface.

Such binary combinations display immiscibility of their components up to1000° C. In these combinations, the less noble metal is one of the groupthat includes the metals Nb, Mo, Ta and W, while the more noblecomponent is a metal that belongs to the group Cu, Ag and Au. Example10, i.e. the Cu--W system, illustrates one of these combinations. TableIII displays representative examples of combinations of binary systemswhich display immiscibility of their components (at at least 1000° C.)and which can be prepared according to this invention.

                  TABLE III                                                       ______________________________________                                                 Nb  Mo           Ta    W                                             ______________________________________                                        Cu         .check mark.                                                                        .check mark. .check mark.                                                                      .check mark.                                  Ag    .check mark. .check mark. .check mark. .check mark.                     Au    .check mark. .check mark. .check mark. .check mark.                   ______________________________________                                    

The rapid formation of bulk quantities of intermetallic compounds atrelatively low temperatures relies on the generation of a largeinterface area between the powder particles (constituent A) and theimmersion deposited coating (constituent B). Compound formation atambient temperature also relies on fast interdiffusion effects thatoccur in several binary combinations. By adjusting the free parametersof the system, i.e. amount of powder, concentration of the metal ions inthe solution, pH of the solution, temperature and duration of theimmersion coating process, the composition of the product material ispre-determined. The composition of compound-forming coated powdersdetermines the structure, and hence the properties of the resultingcompound. Pre-determining the composition of two-phase alloys or metalmatrix composites, allows the custom-design of materials with desiredproperties or combination of properties. By compressing the coatedpowder in appropriate dies, or, alternatively, by the use of processessuch as extrusion forming or injection molding, the product material,i.e., the intermetallic compounds, two-phase alloys or metal matrixcomposites can be formed into near-net shape parts.

Examples of various methods of preparation of powder mixtures that canbe transformed into bulk intermetallic compounds at near ambienttemperature and under moderate pressure are described in the followingnon-limiting Examples 2 to 12. Most examples (2 to 7) are concerned withthe Ag--Sn system which is considered as the paradigm system forapplications as dental restorative materials. An actual typodont filledwith a restorative material prepared according to the methods describedhereby is shown in FIG. 16. A higher magnification optical micrograph ofthe interface area between the dental filling material and toothstructure and illustrating the good bonding is shown in FIG. 17. Finallya photograph of an extracted tooth with a mercury-free filling preparedaccording to the above-described methods is shown in FIG. 18.

Examples 8, 9, 11 and 12 illustrate a more general aspect of theinvention, wherein the inventive processes are applied to form bulkintermetallic compounds in additional systems. Finally, Example 10illustrates the potential of applying some of the processes tonon-compound forming systems in order to achieve homogeneous fine-scalemixtures of different phases.

EXAMPLES

In the Examples which follow wherein a 10% (20%) fluoroboric acidsolution is used, it is prepared by mixing 100 ml (200 ml) ofconcentrated (48%) HBF₄ (ALFA cat. #11484) with 900 (800) ml ofdistilled H₂ O.

Example 1

In this Example the fraction of hard component in the resultant compactis reduced to zero.

Silver powder is stirred for 5 min in a 10% HBF₄ solution andconsolidated in near-net-shape molds for density, compressive andtransverse rupture strength determination. For reference purposes, drysilver powder is also consolidated and its transverse rupture strengthdetermined. The results are shown in FIG. 1 and illustrate significantincrease of transverse rupture strength resulting from the surfacetreatment of the silver powder. This surface treatment increases thecompressive and transverse rupture strength of both spherical anddendritic silver particulates. The density and the compressive strengthof the consolidated silver as a function of the concentration used inthe acid-assisted consolidation process is shown in FIG. 2.

Example 2

An amount of 1 g of -325 mesh atomized Ag₃ Sn compound, 0.9 g of Agpowder, 4-7 μm size, 99.9% (ALFA Cat.#11402) and 0.8 g Ag powder, 1-3μm, 99.9% (ALFA Cat.#11405) and 0.5 g Sn metal powder 99.8% pure, (-325mesh, 12.5 μm average size from CERAC™, Cat. #T-1120) are stirred in 500ml 10% HBF₄₊ 0.2% (NaPO₃)₆, (Fisher, cat. #S-333) for 5 min. The liquidis decanted and the solid residue is compressed into a pellet at 440MPa. X-ray diffraction spectra (FIG. 3, thin line) taken immediatelyafter consolidation reveal the presence of the three components in themixture, namely elemental Ag, Sn and the compound Ag₃ Sn. A second x-raydiffraction pattern (FIG. 3, thick line) obtained after a 19 h stay at37° C., shows an increase of the Ag₃ Sn peaks and a decrease of the Snpeaks. These spectra prove formation of the intermetallic compound Ag₃Sn at near room temperature.

Example 3

A solution of 12.10 g/L AgBF₄ (Aldrich, Cat. No. 20,836-1) in 20% HBF₄is prepared. 8.02 grams of Sn metal powder, 99.8% pure, (-325 mesh, 12.5μm average size from CERAC™, Cat. #T-1120), is added to the solution andstirred for 50 min. The powder is allowed to settle, the liquid removedand the remaining slurry is rinsed in a 10% HBF₄ solution. The powder isallowed to settle again and the slurry removed. A pellet is pressed fromthe slurry, at 1,178 MPa in a steel mold. X-ray diffraction analysis ofthis pellet started within 30 min of its preparation. The diffractionpattern that is obtained is shown in FIG. 4a. Analysis of thediffraction pattern reveals the presence of elemental Sn, elemental Agand an appreciable fraction, approximately 30%, of intermetalliccompound (a mixture of Ag₄ Sn and Ag₃ Sn). The diffraction lines of thecompound are broadened as a result of: (1) a spread of composition, dueto the width of the composition range corresponding to this compound(see Ag--Sn phase diagram); (2) its formation by means of solid stateinter-diffusion between elemental Ag and elemental Sn. In addition, thediffraction lines of the compound Ag₄ Sn overlap partly those of the Ag₃Sn compound, causing further line broadening. FIG. 4b shows thediffraction pattern of the same sample after it had been kept at 37° C.for 8 days. The intensity of some of the diffraction peaks of elementalSn of type (hk1=0) decreased, while the diffraction lines of type(hk1≠0) increased, the intensity of all diffraction peaks due toelemental Ag decreased and those due to the intermetallic compounds Ag₃Sn and Ag₄ Sn increased and narrowed. These results prove that thereaction between elemental Sn and elemental silver proceeded at 37° C.and that, in parallel, the texture of the elemental Sn underwent somechanges. Noteworthy is also the narrowing of the diffraction lines ofthe intermetallic compound, reflecting its increased homogenization.Apparently, the thickness of the compound layer that had formed at theSn--Ag interfaces in the course of the first 8 days following the samplepreparation impedes further compound formation. Indeed close examinationof FIG. 4b shows that some elemental tin and some elemental silver werestill present in that sample after 30 days.

The rupture strength of one sample prepared from the powder/slurry,measured by means of the three point bending test, was 165±5 MPa.

FIG. 5 is a micrograph of a sample that had been hand consolidated usingcommon dental office tools. The light areas represent the residualelemental Sn particles which are embedded in the gray matrix consistingof the intermetallic compound that was formed at the Sn--Ag interfaces.

Example 4

A solution of 20 g/l of AgNO₃ in 10% HBF₄ was prepared. 6.1 g ofatomized, average size 13.5 μm diameter Ag--Sn compound powder was addedto and stirred in the solution at room temperature for 6 min. Thenominal composition of the atomized Ag--Sn alloy was 73 weight % Ag and27 weight % Sn corresponding to the Ag₃ Sn compound composition.However, as a result of fast cooling during the atomization process, theatomized spherical particles were not in a thermodynamic equilibriumstate and contained some elemental Sn. The powder was allowed to settlein the liquid which was removed from above the slurry. The slurry wasrinsed in a 10% HBF₄ solution and again the liquid removed. Some of theslurry was consolidated by compression at 1,178 MPa in a steel mold.

The diffraction pattern of the compressed sample, FIG. 6a revealsapproximately equal amounts of elemental Ag and of the Ag₃ Sn+Ag₄ Sncompounds and some traces of elemental Sn. FIG. 6b shows the diffractionspectrum of the same sample after 64 h at 37° C. This spectrum showsincreased intensity of the diffraction lines corresponding to thecompound, decrease of the Ag lines and almost complete disappearance ofthe diffraction lines of elemental Sn.

FIG. 7 is a metallographic cross-section of a hand consolidated sample,showing a uniform dispersion of the dark gray (the compound) phase andlight (elemental) silver. The spherical shape of the initial pre-alloyedAg--Sn compound particles is retained in most cases.

Example 5

A solution of 12 g/L AgBF₄ (Aldrich, Cat. No. 20,836-1) in 20% HBF₄ wasprepared. An amount of 6.1 grams of atomized Ag--Sn alloy powder wasstirred in the solution, for 5 min. The nominal composition of the alloywas 60 weight % Ag and 40 weight % Sn, corresponding to the two phaseregion, Ag₃ Sn+Sn in the Ag--Sn phase diagram. In other words, the alloycontained a certain fraction (≈18%) of excess Sn. The powder was allowedto settle, the liquid removed and the remaining slurry rinsed in a 10%HBF₄ solution. Again the powder was allowed to settle and the slurryremoved. A pellet was pressed at 1,178 MPa from that slurry in a steelmold.

The x-ray diffraction pattern of this sample taken after it had beenkept for 19 h at room temperature, is shown in FIG. 8a and reveals thepresence of elemental Ag (≈65%), Ag₃ Sn compound (≈35%) and traces ofelemental Sn. The diffraction pattern of the sample, after it had beenkept at 37° C. for 119 h, is shown in FIG. 8b. One observes asignificant increase of the intensity of lines corresponding to the Ag₃Sn compound, decrease of the intensity of the Ag lines, and a littlechange in the very low intensity of lines corresponding to the residualfree Sn. Again, holding the sample at 37° C. induced a significantnarrowing of the diffraction lines corresponding to the intermetalliccompound Ag₃ Sn. The metallography of a hand consolidated sample bycommon dental tools, shown in FIG. 9, shows a structure similar to thatshown in FIG. 5, but with a higher Ag/Ag₃ Sn ratio. The gray areasrepresent the original Ag--Sn alloy particles which are embedded in thebright matrix consisting mainly of silver and silver-rich tin solidsolution.

Example 6

A solution of 265 g/L KI (potassium iodide, ALFA Cat. #11601) and 10 g/LAgNO₃ (silver nitrate, ALFA Cat. #11414) was prepared. The pH=1.0 wasadjusted by HCl and KOH. 4.06 g of Ag--Sn atomized alloy powder (13.2 μmaverage size) were rinsed in 10% HBF₄, and stirred in the first solutionfor 32 min. The nominal composition of the alloy was 60 weight % Ag and40 weight % Sn, corresponding to the two phase region, Ag₃ Sn+Sn in theAg--Sn phase diagram. In other words, the alloy contained a certainfraction (≈18%) of elemental Sn. The powder was allowed to settle, thesolution was decanted and the slurry was rinsed three times in asolution of 2% KI. The solution was decanted and the slurry rinsed inwater, the water was decanted and the slurry rinsed in a 10% HBF₄solution. A pellet was prepared from the slurry by compressing in asteel mold at 1,178 MPa. The consolidated sample was examined by x-raydiffraction 24 h after the initial treatment. The results are shown inFIG. 10. FIG. 10a, is the diffraction pattern of the initial Ag--Snalloy, prior to coating, showing the presence of the intermetalliccompound Ag₄ Sn+Ag₃ Sn and of elemental Sn. FIG. 10b, shows the x-raydiffraction pattern of the sample having undergone the proceduredescribed in this example. The diffraction lines corresponding to theelemental Sn decrease to a low fraction of their initial value,diffraction peaks corresponding to elemental silver are present in thepatterns and the diffraction peaks of the intermetallic compound broadenconsiderably. The broadening is due to the formation of additional Ag₃Sn+Ag₄ Sn compounds by means of the interdiffusion reaction of Ag withthe free Sn. As mentioned previously, both intermetallic compounds existover a range of compositions that corresponds to a range of latticeparameters, giving rise to the broadened diffraction lines.

FIG. 11 is an optical micrograph of a pellet of that slurry, compressedat 470 MPa, illustrating the efficiency of the process, described inthis example, in producing with a relatively low volume fraction ofsilver (represented by the bright circular halos, surrounding thespherical, atomized Ag--Sn alloy particles), large silver--silverinterface area that promotes consolidation, and silver-tin interfacethat promotes compound formation.

Example 7

A silver coating solution was prepared by dissolving 265 g of KI (ACS,ALFA Cat. #11601) and 10 g of AgNO₃ (ACS, ALFA Cat. #11414, 99.94% pure)and 8 ml concentrated HCl (Mallinckrodt) in 1 L of distilled H₂ O at pH1.06 after adjustment by 10 M KOH. An amount of 4 g atomizedAg(60)Sn(40) powder with 13.2 μm average particle size was stirred in500 ml of 10% HBF₄ solution for 30 sec. The acid was decanted and thewet powder rinsed in distilled H₂ O. The powder was stirred in 500 ml ofthe silver coating solution for 30 min. After the slurry settled down,the solution was decanted and the slurry rinsed in 2% KI solution fourconsecutive times and finally rinsed with distilled H₂ O. An amount of0.81 g of Ag(70)Cu(30) atomized powder, 11.5 μm average size, 3.00 g ofAg powder, 4-7 μm size, 99.9% (ALFA Cat.#11402) and 1.48 g Ag powder,1-3 μm, 99.9% (ALFA Cat.#11405) were mixed with the silver coatedAg(60)Sn(40) wet powder in 500 mL of 10% HBF₄. The slurry afterdecantation was consolidated in a steel mold at 471 MPa.

FIG. 12 is a metallography of the consolidated sample. The silver copperalloy component was added to increase the overall hardness of theconsolidated solid.

Example 8

A solution of 21.7 g/L AgBF₄ (Ag as metal 12 g) (Aldrich, Cat. No.20,836-1) in 5% HBF₄ was prepared. An amount of 7 g of In metal powder,˜400 mesh particle size, (ALFA, 99.99%, Cat. #11024) was stirred for 8min in 250 ml of the silver fluoroborate solution at a pH of 0.5 and 23°C. temperature. The slurry, after decanting the solution, was rinsed in10% HBF₄ and consolidated at 300 MPa in a steel mold.

In FIG. 13, curve (a) is the x-ray diffraction pattern of the resultingdisc, obtained approximately 1 h after the removal of the slurry fromthe solution. Analysis of the pattern obtained at this stage shows thatthe compound AgIn₂ (Φphase) is the main constituent with some additionalAg₉ In₄ (γ phase), AgIn (ζ phase), an Ag based phase (a solid solution)and an In-based phase. Curve (b) shows the x-ray pattern obtained fromthe same sample after an anneal of 142 d at ambient temperature. Therelative intensity of the diffraction lines indicated that the AgIn₂ isthe main constituent, making up approximately 90% of the material.

Example 9

Gold chloride (AuCl) was prepared by dissolving metallic gold in aquaregia and evaporating the solution. An amount of 2.65 g gold chloride(AuCl) was dissolved in 250 ml 10% fluoroboric (HBF₄) acid at pH=0.2,and 23° C. An amount of 4.03 g of Sn powder, 1-5 μm particle size wasadded to 250 ml solution and stirred for 30 min. The solution wasdecanted in the slurry rinsed in 10% HBF₄.

FIG. 14 is a x-ray pattern of a disc made from the slurry that wasremoved from the solution consolidated at 1220 MPa. The x-ray exposurewas made lh after the removal of the slurry from the solution. Thesample consists mostly of the AuSn compound, a small fraction of AuSn₂and some excess Au.

Example 10

A plating solution was prepared by dissolving 288 g of potassiumpyrophosphate K₄ P₂ O₇ (ALFA, Johnson Matthey, Cat.#13436), 75 g ofcopper pyrophosphate Cu₂ P₂ O₇.3H₂ O (ALFA, Johnson Matthey,Cat.#18220), 10 g potassium nitrate KNO₃ (J. T. Baker, Cat. #3190-1) and5 ml of ammonium hydroxide NH₄ OH (Mallinckrodt, Cat. #1177) in 1000 mlof distilled water. The pH of the solution was adjusted by adding,ammonium hydroxide or potassium hydroxide to increase, phosphoric acidH₃ PO₄ to decrease to pH=8.65. 175 ml of that solution (containing 4.7 gof copper as metal) were heated to 55° C. and 30.5 g of tungsten (W)particles, 2 to 28 μm size range added. The suspension was stirred untilthe Cu was exhausted as determined by the change of color of thesolution. The time of processing was dependent on the particle size(surface area). The copper was exhausted within 15 minutes for 2 μm sizetungsten particles and 60 minutes for 28 μm size particles. The liquidwas decanted, the slurry rinsed several times in water and ethanol, andair dried. The dry powder was mixed with 2 vol % of fluoroboric acid(HBF₄) and compressed in steel molds at pressures in the 1500-1800 MParange. FIG. 15 shows a micrograph of a sample that had been cold-pressedat 1200 MPa from a slurry removed from the solution. The density of thiscompact was 88±2% of theoretical density.

Example 11

A solution of 29.4 g/L AgBF₄ (16.2 g Ag as metal) (Aldrich, Cat. No.20,836-1) in 5% HBF₄ was prepared. An amount of 3.15 g of In metalpowder, ˜400 mesh particle size, (ALFA, 99.99%, Cat. #11024) was stirredfor 10 min in 250 ml of the silver fluoroborate solution at a pH of 0.5and 23° C. temperature. The slurry, after decanting the solution, wasrinsed in 10% HBF₄ and consolidated at 300 MPa in a steel mold.

An x-ray pattern of the resulting disc was obtained approximately 20 hafter the removal of the slurry from the solution. Analysis of thepattern obtained shows that Ag is the main component with the additionalpresence of AgIn₂ (Φ phase), Ag₉ In₄ (γ phase), and AgIn (ζ phase). Thex-ray pattern after a 51 h anneal at 140° C. shows that Ag₉ In₄ is themajor constituent (75%) with Ag₃ In (25%) and some residual Ag andtraces of AgIn₂.

Example 12

A solution of 708 g/L Cu(BF4)2 (Fidelity Chemical Products Corp. #0360)in 10% HBF₄ was prepared. Silver metal constituted 190 g of that amountof copper fluoroborate. An amount of 8 g of Sn metal powder, -325 meshparticle size, was stirred for 10 min in 100 ml of the copper added to400 ml solution of 10% fluoroborate solution at a pH 0.2 and at 23° C.temperature. The slurry, after decanting the solution, was rinsed in 10%HBF₄ and consolidated at 300 MPa in a steel mold.

X-ray exposure was made within 1 h after the removal of the slurry fromthe solution. Analysis of the pattern obtained at this stage shows thatCu and Sn are the main components with some additional Cu₃ Sn andCu₅.6Sn (B'-phase) present. After aging for 69 H at 150° C., the sampleconsists of mixture of Cu and Cu₃ Sn and traces of another phase.

While the invention has been illustratively described herein withreference to various preferred features, aspects and embodiments, itwill be appreciated that the invention is not thus limited, and may bewidely varied in respect of alternative variations, modifications, andother embodiments, and therefore the invention is to be broadly contruedas including such alternative variations, modifications and otherembodiments, within the spirit and scope of the invention claimed.

What is claimed is:
 1. A process for consolidating a powder,particulate, sheet or foil of at least one member selected from thegroup consisting of elemental metallic, metallic alloy and intermetalliccompounds having surface oxides, the process comprising:coating thepowder, particulate, sheet or foil with an oxide replacing metal byimmersing said powder, sheet or foil in a solution containing anelectrolyte, said electrolyte selected from the group consisting offluoroborate, sulfamate, iodide, cyanide, nitrate, pyrophosphate andsulfide salts of the oxide-replacing metal and mixtures thereof;separating the coated powder, particulate, sheet, or foil from theelectrolyte solution; treating the coated powder, particulate, sheet, orfoil with a solution consisting essentially of an acid selected from thegroup consisting of acetic acid, fluoroboric acid, sulfuric acid,fluoric acid, citric acid, adipic acid, ascorbic acid and nitric acid;and consolidating the treated powder, particulate, sheet, or foil atnear ambient temperature by compacting said treated powder, particulate,sheet or foil without amalgamating same.
 2. The process according toclaim 1, wherein the oxide-replacing metal is selected from the groupconsisting of Au, Ag, Fe, Pt, Pd, Ni, Co and Cu.
 3. The processaccording to claim 1, wherein the elemental metallic is selected fromthe group consisting of Ga, Ir, La, Re, Rh, Ru, Sn, Ti, Y, Zn, Nb, Mo,Ta, Sc, Hf, Ce, Pr, Nd, Sm, Gd, Tb, Sy, Ho, Er, Tm, Yb, Lu, U and W. 4.The process according to claim 3, wherein the oxide replacing metal isCu, the elemental metallic is W and the electrolyte is copperpyrophosphate.
 5. The process according to claim 3, wherein the oxidereplacing metal is Ni and the elemental metallic is Ti.
 6. The processaccording to claim 1, wherein the intermetallic compound is comprised ofat least one first elemental metal being a matrix species, and at leastone second elemental metal being a fast diffusing species which is ableto diffuse into the interstitial space of the matrix species.
 7. Theprocess according to claim 1, wherein the powder comprises atomized,spherical particles having an equiaxial particle size of from about 0.5μm to about 50 μm.
 8. The process according to claim 1, wherein thepowder comprises a mixture of (1) a silver-tin compound selected fromthe group consisting of a pre-alloyed Ag₃ Sn and a pre-alloyed Ag₄ Sn,and (2) at least two elemental metallic powders.
 9. The processaccording to claim 1, wherein the powder further comprises a reinforcingstructural particulate or fiber additive.
 10. The process according toclaim 9, wherein the structural particulate or fiber additive isselected from the group consisting of alumina powder, silicon carbide,graphite, diamond, sapphire and combinations thereof.
 11. The processaccording to claim 1, wherein the acid is fluoroboric acid.
 12. Theprocess according to claim 1, wherein the concentration of the acid isfrom about 1% to about 30%.
 13. A process according to claim 1, whereinsaid powder, particulate, sheet, or foil is consolidated in an acidicsolution.
 14. A process for synthesizing bulk intermetallics orhomogeneously distributed two-phase alloys comprising:generating aninterface between as first powder of a less noble element and a secondmore noble element by dissolving the second element in an electrolyte,immersing the first powder element in the electrolyte, allowing thesecond element to deposit from the electrolyte onto the first powderelement; allowing the first powder element and the second element toform an intermetallic compound or a homogeneously distributed two-phasealloy at or about ambient temperature; separating the coated powder fromthe electrolyte solution; treating the coated powder with a solutionconsisting essentially of an acid selected from the group consisting ofacetic acid, fluoroboric acid, sulfuric acid, fluoric acid, citric acid,adipic acid, ascorbic acid and nitric acid; and consolidating thetreated powder without amalgamating same.
 15. The process according toclaim 14, wherein the first element is selected from the groupconsisting of Sc, Y, Ln, Ti, Zr, Th, U, Np and Pu and the second elementis selected from the group consisting of Fe, Co, Ni, Pd and Pt.
 16. Theprocess according to claim 15, further comprising controllingconcentration of electrolyte, amounts of electrolyte solution and solidcomponent and time-duration of the immersion so as to controlcomposition ratio of the intermetallic formed.
 17. The process accordingto claim 14, wherein the first element is selected from the groupconsisting of Nb, Mo, Ta and W and the second element is selected fromthe group consisting of Cu, Ag and Au.
 18. The process according toclaim 17, wherein a homogeneously dispersed two-phase alloy is formed.19. The process according to claim 14, wherein the first element isselected from the group consisting of In, Ti, Sn and Pb and the secondelement is selected from the group consisting of Pd, Cu, Ag and Au. 20.The process according to claim 19, further comprising adding achemically inert fine dispersion component to the electrolye therebyforming a metal matrix component wherein the chemically inert componentis homogeneously dispersed in the matrix.
 21. The process according toclaim 20, wherein the chemically inert fine dispersion component isselected from the group consisting of alumina powder, silicon carbide,graphite, diamond, sapphire and combinations thereof.
 22. The processaccording to claim 19, further comprising controlling concentration ofelectrolyte, amounts of electrolyte solution and solid component andtime-duration of the immersion so as to control composition ratio of thetwo phase alloy formed.
 23. The process according to claim 14, wherein atwo phase alloy is formed and the first element is aluminum and thesecond element is selected from the group consisting of Zn, Sn, Cr, Feand Ni.
 24. The process according to claim 14, further comprisingapplying axial pressure or injection molding to consolidate the bulkintermetallic or two-phase alloy into near net shape.